Respiratory organs and the human respiratory system

Breathing and respiratory organs

In plain words: most body tissues need a steady oxygen supply and release carbon dioxide during metabolism. Mature human RBCs are an exception: they carry oxygen but lack mitochondria and do not consume it in aerobic respiration. Breathing is the delivery and waste-collection service: the lungs bring air close to the blood, and the blood carries the gases between the lungs and every cell.

Respiratory organs in animals

The mechanism of breathing varies among animal groups, depending mainly on their habitat and level of organisation (NCERT Section 14.1, page 183):

This table gives NCERT’s broad patterns, not rules without exceptions; for example, it already lists terrestrial insects using tracheae.

Gills are special vascularised structures, and lungs are vascularised bags (page 183). See the frog’s cutaneous and pulmonary respiration.

Human respiratory system

The air passes through these parts in order (NCERT Section 14.1.1, pages 184–185):

1. A pair of external nostrils, opening above the upper lip, leads through the nasal passage into the nasal chamber.
2. The nasal chamber opens into the pharynx, part of which is the common passage for food and air.
3. The pharynx opens through the larynx into the trachea. The larynx is a cartilaginous box that helps in sound production, so it is called the sound box. During swallowing, the glottis can be covered by a thin, elastic, cartilaginous flap, the epiglottis, which stops food entering the larynx.
4. The trachea is a straight tube running to the mid-thoracic cavity, where it divides, at the 5th thoracic vertebra in NCERT’s account, into the right and left primary bronchi.
5. Each bronchus divides repeatedly into secondary and tertiary bronchi and bronchioles, ending in very thin terminal bronchioles. NCERT includes the trachea, bronchi and “initial bronchioles” under incomplete cartilaginous rings. Modern histology distinguishes them: tracheal rings give way to bronchial cartilage plates, and bronchioles lack cartilage (StatPearls, “Histology, Lung” and “Histology, Respiratory Epithelium”).
6. Beyond NCERT’s shortened sequence, terminal bronchioles lead to respiratory bronchioles, alveolar ducts and alveolar sacs containing thin-walled alveoli. Respiratory bronchioles already have alveoli in their walls (StatPearls, “Histology, Lung”).

Modern anatomical descriptions place the carina around T4–T5 rather than treating T5 as an invariant level (StatPearls, “Anatomy, Head and Neck, Trachea”).

The branching network of bronchi, bronchioles and alveoli makes up the lungs. The two lungs are covered by a double-layered pleura, with pleural fluid between the layers, which reduces friction on the lung surface. The outer pleural membrane is in close contact with the thoracic lining, and the inner one with the lung surface (pages 184–185).

The lungs sit in the thoracic chamber, an anatomically air-tight chamber formed dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs and below by the dome-shaped diaphragm. Normally, pleural coupling transmits chest-wall movement to the lungs, changing their volume. The airway remains open to the outside; “air-tight” describes the intact surrounding chest/pleural arrangement, not a sealed airway. This arrangement is essential for breathing, because we cannot alter the pulmonary volume directly (NCERT page 185).

Conducting and respiratory parts

The part starting with the external nostrils up to the terminal bronchioles constitute the conducting part whereas the alveoli and their ducts form the respiratory or exchange part of the respiratory system.

NCERT Biology (Class 11), Section 14.1.1, page 185

• The conducting part transports atmospheric air to the alveoli, clears it of foreign particles, humidifies it and brings it to body temperature (page 185).
• The exchange part is the site of diffusion of O_₂ and CO_₂ between blood and alveolar air (page 185). NCERT’s quoted list is abbreviated; modern anatomy includes respiratory bronchioles with alveoli as well as alveolar ducts, sacs and alveoli.

Steps of respiration

NCERT lists five steps (page 185):

1. Breathing or pulmonary ventilation: atmospheric air is drawn in and CO_₂-rich alveolar air is released out.
2. Diffusion of O_₂ and CO_₂ across the alveolar membrane.
3. Transport of gases by the blood.
4. Diffusion of O_₂ and CO_₂ between blood and tissues.
5. Use of O_₂ by the cells for catabolic reactions, with the release of CO_₂: cellular respiration, dealt with in Chapter 12.

Solved examples

Common mistakes to avoid

Quick revision summary

• Breathing is the exchange of atmospheric O_₂ with the CO_₂ produced by cells. Lower invertebrates use the body surface, earthworms the moist cuticle, insects tracheal tubes, most aquatic arthropods and molluscs and typical fishes gills, and typical adult land vertebrates lungs; frogs also use their skin.
• Human airway: nostrils → nasal chamber → pharynx → larynx (sound box; epiglottis) → trachea → bronchi (at the 5th thoracic vertebra) → bronchioles → alveoli.
• Lungs are covered by a double-layered pleura with pleural fluid, inside an air-tight thoracic chamber closed below by the diaphragm.
• Conducting part: nostrils to terminal bronchioles (transport, clean, humidify, bring to body temperature). Exchange part: the alveolated respiratory region, including respiratory bronchioles in modern anatomy.
• Steps: breathing (pulmonary ventilation), diffusion at the alveoli, transport by blood, diffusion at the tissues, use of O_₂ by cells.

Questions students ask

What is the difference between breathing and respiration?

Breathing is the physical movement of air into and out of the lungs, while cellular respiration is the chemical oxidation of food inside the cells that releases energy. Breathing, or pulmonary ventilation, is done by muscles that change the volume of the thorax, about 12–16 times a minute in a healthy human (NCERT pages 185–186); cellular respiration goes on in the cytoplasm and mitochondria and traps the energy as ATP (NCERT Chapter 12, page 154). The two are linked: breathing supplies the O_₂ that the cells use and removes the CO_₂ they make (NCERT page 183).

What is the difference between terminal bronchioles and respiratory bronchioles?

Terminal bronchioles are the last tubes of the conducting part and carry no alveoli, while respiratory bronchioles, which NCERT does not name, are the narrower branches after them whose walls already carry some alveoli and so take part in gas exchange. NCERT ends the conducting part at the terminal bronchioles and says each terminal bronchiole gives rise to a number of alveoli (NCERT pages 184–185). Beyond the NCERT text, terminal bronchioles lead into several orders of respiratory bronchioles, then alveolar ducts and alveoli (MeSH, US National Library of Medicine, “Bronchioles”; StatPearls, “Histology, Lung”). The conducting/respiratory boundary is still at the end of the terminal bronchioles; the modern description fills in structures NCERT omits.

How is a frog’s respiration different from a human’s?

A frog uses more than one respiratory surface, while a human relies on its lungs: in water a frog respires through its skin (cutaneous respiration), and on land its buccal cavity, skin and lungs all act as respiratory organs (NCERT Chapter 7, page 82). During aestivation and hibernation the frog exchanges gases through the skin (page 82), whereas in humans the lungs alone serve, sitting in an air-tight thorax closed below by the diaphragm (NCERT pages 184–185). NCERT Section 7.2.2 on the frog covers its cutaneous and pulmonary respiration, including its lungs (page 82).

Why are the cartilaginous rings of the trachea incomplete?

The trachea’s cartilaginous rings are incomplete so that they hold the windpipe open while leaving a soft back wall next to the oesophagus, a reason NCERT does not give: it describes them only as incomplete cartilaginous rings that support the airway (NCERT page 184). Beyond the NCERT text, the trachea has 16 to 20 C-shaped rings of hyaline cartilage, open at the back, that stop it collapsing during inspiration. Their open ends are bridged by a smooth muscle, the trachealis, so the back wall gives way as the oesophagus expands during swallowing, and the muscle narrows the airway during coughing (StatPearls, “Anatomy, Head and Neck, Trachea”).

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. What is the function of the pleural fluid?

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   It reduces friction on the lung surface (NCERT page 184).

2. Q2. Which structure is called the sound box?

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   The larynx, a cartilaginous box that helps in sound production (NCERT page 184).

3. Q3. Name the four structures that form the walls of the thoracic chamber.

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   The vertebral column (dorsally), the sternum (ventrally), the ribs (laterally) and the dome-shaped diaphragm (below) (NCERT page 185).